We intend to study the mode of action of a regulatory protein found in mitochondria, called the inhibitor protein. This low molecular weight protein inhibits the hydrolysis but not the synthesis of ATP by the enzymes which catalyze oxidative phosphorylation. We are particularly interested in understanding how Mg ATP promotes the binding of inhibitor protein to the terminal coupling factor of oxidative phosphorylation and how Mg ADP promotes dissociation of the protein. We also intend to study the mode of inhibition by the antibiotic efrapeptin of ATP synthesis and hydrolysis. These studies may serve as a test of the "one-half-the-sites reactivity" recently proposed for oxidative phosphorylation. We will continue our studies of possible changes in binding affinities of adenine nucleotides at the catalytic site accompanying oxidative phosphorylation.